Unsolved Problems in Lunar Science

It is clear from the foregoing discussion that although a framework for the formation of the Moon, consistent with the available astronomical, physical, chemical, geological and chronological data has been developed, much remains to be understood about its origin and early evolution. It is desirable to identify problems with the current models and gaps in our understanding, so that goals for future missions to the Moon can be defined. Such an exercise cannot be exhaustive and, at the same time, would be subjective. Bearing these limitations, we enumerate some interesting points here. The giant impact hypothesis, though able to explain most of the observations, appears to be ad hoc. It also considers only two bodies in isolation, a large impactor and the earth, whereas some debris from previous collisions on the earth and several moonlets may be already existing at the time of the terminal giant impact. Their role in the formation of the Moon remains to be ascertained. There are also questions related to the magma ocean. The extent of magma ocean is not known, although there are indications that less than half of the Moon was involved. The question whether the Moon has a core or not, and its size and composition, has been extensively debated, but there are no seismological observations which can provide a direct and conclusive answer.

The formation of the Moon’s core is also related to the size of the magma ocean. The extremely high concentration of incompatible elements (KREEP, U, Th, etc.) in the Procellarum–Imbrium–Frigoris regions on the near-side of the Moon and their relatively low concentrations in the SPAR on the far side (Figures 1 and 2) have a bearing on the extent and the homogeneity of the magma ocean, chemical inhomogeneities of the crust and chemical processes responsible for formation of the large differentiated regions. Opinions are widely divided on the LHB episode. Does it really represent a peak in lunar cratering history or is it simply the tailing of the frequency of accretionary impacts? Studies of large basins on Mercury, Mars and asteroids have raised the question of this peak in cratering frequency: whether it was solar system-wide, confined to the inner planets or only to the Moon24? The LHB was caused by planetesimals moving in heliocentric orbits or by moonlets in geocentric orbits. The composition of impactors25 would be useful in understanding the earth’s accretionary history and the nature of the source material as well. Period and mechanism of the formation of some large mare basins, e.g. South Pole Aitken basin, is not understood. It requires about a 1000-km impactor with low relative velocity to form this basin. It was probably formed about 4.2 Ga and may still have some surviving original impact melt material. A sample return mission to SPAR should be informative. The lunar cataclysm has catastrophic consequences for living organisms on the earth. The fossil records and biogenic isotopes found in terrestrial rocks show that life may have existed on the earth as early as 3.8 Ga and water as early as 4.2 Ga, well within the time span covered by large impacts on the earth–Moon system. While large impacts occurred on the Moon (4.2–3.8 Ga), how could life evolve or survive on the earth, which should have been subjected to at least ten times more impacts, so energetic that the oceans and environment on the earth would be sterilized? Since the time bracket of LHB is based on only a few breccias, the cataclysmic era is not well constrained. It is important to  define the cut-off time of large impacts on the Moon as well as on the earth from the point of view of the biological evolution. For this reason, samples of the far side of the Moon are very crucial. Some of the problems stated above can be resolved by long cores (several km) from selected sites on the Moon. Long cores through the bedrock can provide information about stratigraphic relations, composition of the lunar interior and heat flow (which depends on the radioactive content). A soil core going to the bedrock can provide insight into the nature of the solar activity (solar wind, heavy nuclei, solar energetic particles), way back in time when the lunar crust had just formed. Their flux and isotopic composition may be related to the nuclear reactions in the sun. The 2-m long Apollo 15 core represented roughly 1 Ga of records and therefore extending it to 4.5 Ga may not be difficult. However, taking long cores through soil or bedrock may pose challenging technical problems. It is, therefore, necessary to explore scientific programmes that are realizable and inexpensive. Recent studies have raised questions regarding the various hypotheses related to early lunar chemical processes. For example, the thickness of individual flows within a basin is not well determined. Volcanism on the far side of the Moon and its chronology still remain to be quantified. It is believed that filling of mare basins was a prolonged process which took several hundred million years. The onset and termination of lava-filling should be precisely determined. We ought to know where the youngest and oldest basalts on the moon are exposed and what are their absolute age, composition and stratigraphy. An important objective, therefore, could be to get a timestratigraphic sequence from the regolith as well as from mare basalts. The mineralogical stratigraphy of lunar mare basalts and crust is crucial for understanding the evolution of the Moon. The best current imagery suitable for crater counts is that of Lunar Orbiter and Apollo missions, but is not available for the entire Moon. The data are particularly lacking for the far side. Stratigraphic investigations require high-resolution imaging under relatively low sun angle, so that crater rims and flow fronts can be clearly identified. On the other hand, mineralogical studies need high sun angle so that shadows can be avoided. Clementine and Lunar Prospector have provided global mineralogical and chemical data27,28, but superior data with high spatial resolution can be obtained with new sensors with high spectral resolution available now.